DESCRIPTION (Verbatim from applicant's abstract): The long-term objective of this project is to determine the functions of glial cells (Muller cells and astrocytes) in the mammalian retina. It is widely recognized that glial cells have important support functions in the retina, including uptake of neurotransmitters and regulation of extracellular potassium and pH. The role of glial cells in direct modulation of neuronal activity is not yet understood, however. In the preceding project period, we demonstrated that intercellular Ca2+ waves can be propagated through glial cells in the rat retina and that these glial Ca2+ waves modulate spike activity in neighboring neurons. In the proposed project period, we will extend our studies of glial modulation of neuronal activity and explore additional aspects of glial Ca2+ signaling with the goal of determining the significance of these interactions in vivo. The specific aims for the project period are: (1) to identify natural stimuli that elicit Ca2+ signals in retinal glial cells; the hypotheses to be tested are that: (a) chemicals released under normal or pathological conditions evoke glial Ca2+ increases, and (b) light stimulation evokes glial Ca2+ increases; (2) to test the hypothesis that spontaneous Ca2+ oscillations in glial cells modulate the activity of neighboring neurons, using regression analysis to correlate neuronal spike activity and membrane potential with Ca2+ levels in adjacent glial cells displaying spontaneous Ca2+ oscillations; (3) to characterize mechanisms of glial cell modulation of neuronal activity; the hypotheses to be tested are that: (a) excitatory neuronal modulation is mediated by release of glutamate from glial cells onto neurons, and (b) inhibitory modulation is mediated indirectly by glial activation of inhibitory amacrine cells; (4) to elucidate the mechanism by which Ca2+ waves are propagated in retinal glial cells; the hypothesis to be tested is that wave propagation is mediated by the release of ATP, which functions as an extracellular messenger; and (5) to characterize physiological changes in retinal glial cells elicited by propagation of Ca2+ waves; the hypotheses to be tested are that: (a) Ca2+ increases modulate inward rectifier potassium and Ca2+-dependent potassium conductances, and (b) Ca2+ increases generate intracellular pH variations in retinal glial cells. Glial cells have been implicated in many types of retinal pathology, including diabetic retinopathy, glaucoma, and macular degeneration. Knowledge of the basic physiological properties of retinal glial cells and their interactions with retinal neurons will add to our understanding of how these cells contribute to retinal pathology. The research outlined in this application will provide significant progress towards this goal.